Stoichiometric lithium cobalt oxide and method for preparation of the same

ABSTRACT

The present invention provides a LiCoO 2 -containing powder comprising LiCoO 2  having a stoichiometric composition via heat treatment of a lithium cobalt oxide and a lithium buffer material to make equilibrium of a lithium chemical potential therebetween; a lithium buffer material which acts as a Li acceptor or a Li donor to remove or supplement Li-excess or Li-deficiency, coexisting with a stoichiometric lithium metal oxide; and a method for preparing a LiCoO 2 -containing powder. Further, provided is an electrode comprising the above-mentioned LiCoO 2 -containing powder as an active material, and a rechargeable battery comprising the same electrode. 
     The present invention enables production of a LiCoO 2  electrode active material which has improved high-temperature storage properties and high-voltage cycling properties, and is robust in composition fluctuation in the production process. Therefore, the present invention provides advantages such as reduction of time and labor required for quality control and process management in the mass-production of the electrode active material, and decreased production costs of LiCoO 2 .

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 11/688,636, filed on Mar. 20, 2007, which is adivisional application and claims priority to Korean Patent ApplicationNo. 10-2006-0025116 filed on Mar. 20, 2006, and Korean PatentApplication No. 10-2006-0040969 filed on May 8, 2006, in the KoreanIntellectual Property Office, the disclosures of which are incorporatedherein in their entirety by reference.

FIELD OF THE INVENTION

The present invention relates to a lithium cobalt oxide having astoichiometric composition, which can be used as a cathode activematerial for lithium rechargeable batteries.

BACKGROUND OF THE INVENTION

A report in 1980 that LiCoO₂ is useful for a cathode active material oflithium rechargeable batteries was followed by a lot of research, soLiCoO₂ was adopted by commercial enterprises as a cathode activematerial for lithium rechargeable batteries. But, the high cost ofLiCoO₂ contributes significantly (about 25%) to the cost of the batteryproduct. High competition presses producers of rechargeable lithiumbatteries to lower the cost.

The high cost of LiCoO₂ is caused by two reasons: First, the high rawmaterial cost of cobalt, and second the high cost of establishingreliable quality management and ensuring perfect process control duringlarge scale production.

Especially, the quality management and process control aim to achievehighly reproducible products having optimized properties, where theperformance of every batch fluctuates very little from those optimumproperties. High reproducibility and little fluctuations of theperformance of LiCoO₂ are absolutely essential in currenthighly-automated high volume lithium battery production lines.

A major problem is that LiCoO₂ is a sensitive material. Small changes ofproduction process parameters cause large fluctuations of theperformance of the cathode product. So quality management and processcontrol require much effort and high costs.

LiCoO₂ is a stoichiometric phase. Under normal conditions (for example800° C. in air) no reliable indication for any Li:Co non-stoichiometryhas been reported in the literature.

Only stoichiometric LiCoO₂ with a Li:Co ratio very near to 1:1 hasproperties which are suitable for the cathode active material of thecommercial lithium batteries. If the Li content is higher than 1:1,LiCoO₂ will coexist with a secondary phase which contains the excesslithium and largely consists of Li₂CO₃. Li₂CO₃ impurities in thecommercial LiCoO₂ cathode active material are highly undesirable. Suchsamples are known to show poor storage properties at elevatedtemperature and voltage. One typical test to measure the storageproperties is storage of fully charged batteries at 90° C. for 5 hours.

If the cathode contains Li₂CO₃ impurities, this may result in strongswelling (increase of thickness) of polymer cells. Even the muchstronger metal cases of prismatic cells may bulge.

If the Li content is lower than 1:1, then the cathode contains divalentcobalt, i.e. LiCoO₂ coexists with cobalt oxides. Lithium-deficientLiCoO₂ shows poor cycling stability at a high voltage (>4.3 V),especially at an elevated temperature It is speculated that the highercatalytic activity of divalent cobalt present in the cobalt oxide phasesupports the undesired oxidation of an electrolyte on the surface ofLiCoO₂. Alternatively, divalent cobalt might, especially at a highvoltage, dissolve in the electrolyte, and undergo precipitation at theanode side, thereby damaging a solid electrolyte interphase (SEI) layeron the anode.

Only in a lab, it is easy to prepare stoichiometric LiCoO₂ practicallyfree of Li₂CO₃ or CoO_(x) impurities by simple heating of LiCoO₂. Thehigh cycling stability of such cathodes (in coin cells) has beendemonstrated in the literature. It is speculated that the good cyclingstability is attributed to two effects: (1) At small scale (lab sizesamples) the excess lithium (Li₂CO₃) easily evaporates during sintering,and (2) Heating repairs any damage to the surface of LiCoO₂, which wascaused by air exposure, probably by a reductive attack by hydrocarbons.

A similar re-heating of LiCoO₂ is not effective to solve the problemsassociated with the high temperature properties and cycling stabilitywhich may occur in the large scale production. First, largescale-produced LiCoO₂ has not a damaged surface. After production theproduct is usually filled into air tight packaging, so any damage causedby air exposure is practically absent. Second, on a large scale, excesslithium does not evaporate practically. Very small amounts of Li₂CO₃ canbe decomposed because volatile phases exist with very smallthermodynamic equilibrium partial pressure. At small partial pressuregas transport is very slow, so that only tiny amounts of Li₂CO₃ can bedecomposed. If we deal with large samples then the gas transport is notsufficient to decompose significant amounts of Li₂CO₃.

The situation is different if Li₂CO₃ decomposes in the presence of alithium acceptor (such as cobalt oxide). In this case the thermodynamicequilibrium partial pressure is high and the gas transport kinetics isfast enough to decompose Li₂CO₃.

More generally, it is very difficult or even impossible to prepareLiCoO₂ with the exact desired Li:Co ratio at large scale. If an excessof cobalt is used, then a cobalt oxide impurity remains. Unfortunately,small impurities of CoO_(x) are practically impossible to be detected bystandard quality control methods, but they are very important for theperformance of the cathode. If an excess of lithium is used, lithiumimpurities remain due to the low evaporation at large scale. Even if thepremixed (Li₂CO₃ and Co-oxide) powder would exactly have the desiredLi:Co ratio, any inhomogeneity in the mixed powder would after sinteringcreates a powder with regions being lithium-rich and other regions beinglithium-deficient. Additionally some Li₂CO₃ can melt before fullyreacting with the Co-oxide, and the molten Li₂CO₃ would tend to separatedownwards. This will cause a Li:Co gradient with Li-deficient sample atthe top and Li-excess at the bottom of the sintering vessel. As aresult, very small amounts of impurity phases (Li₂CO₃ or Co-oxide) arepresent.

Much previous art to improve properties of LiCoO₂ has been disclosed.Examples of such efforts are surface coating of LiCoO₂, doping of LiCoO₂with other metal cations and the preparation of non-stoichiometricLiCoO₂ at a very high temperature. Each effort created some satisfactoryresults, but the results are not enough for a mass production process,and make another problem of the costs of additional processes.

SUMMARY OF THE INVENTION

The invention discloses that a property fluctuation of LiCoO₂ in amass-production process is primarily caused by the difference of lithiumchemical potential of LiCoO₂, and robust LiCoO₂ less sensitive toprocess parameters can be prepared by co-firing LiCoO₂ and a solid statelithium buffer material to adjust a stoichiometric composition oflithium and cobalt to a desired range.

Conventional LiCoO₂ that is mass-produced has a problem in that littledeviation of the composition from the desired stoichiometry results insignificant fluctuation of a lithium chemical potential. If the lithiumchemical potential is fixed within a given range, it is possible toachieve excellent high-voltage cycling properties and storage propertiesof LiCoO₂ even under high temperature conditions on the mass productionscale.

Therefore, the present invention has been made in view of the aboveproblems, and it is an object of the present invention to provide apowder comprising LiCoO₂ having a stoichiometric composition via heattreatment of a lithium cobalt oxide and a lithium buffer material tomake equilibrium of the lithium chemical potential therebetween, amethod for preparing the same, the above-mentioned lithium buffermaterial, an electrode comprising the above-mentioned LiCoO₂-containingpowder as an active material, and a rechargeable battery comprising thesame electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a thermodynamic schematic diagram of lithium chemicalpotential of LiCoO₂;

FIG. 2 is a thermodynamic schematic diagram of lithium chemicalpotential of Li buffer (LiMO₂);

FIG. 3 is a thermodynamic schematic diagram which shows an equilibriumstate of lithium chemical potential between Li-excess LiCoO₂ and Libuffer (LiMO₂);

FIG. 4 is a thermodynamic schematic diagram which shows an equilibriumstate of lithium chemical potential between Li-deficient LiCoO₂ and Libuffer (LiMO₂);

FIG. 5 is a graph showing cycling stability of LiCoO₂ at a differentLi:Co ratio in Reference Example 1;

FIG. 6 is a crystallographic map of solid state lithium bufferLi(Mn_(1/3)Ni_(1/3)Co_(1/3))O₂ in Reference Example 4;

FIG. 7 is a graph showing cycling stability of solid state lithiumbuffer Li(Mn_(1/3)Ni_(1/3)Co_(1/3))O₂ in Reference Example 5;

FIG. 8 is an FESEM (Field Emission Scanning Electron Microscope) imagewhich shows (a) LiCoO₂ precursor powder, and (b) TR01 sample obtained byco-firing of 90% LiCoO₂ and 10% Li(Mn_(1/3)Ni_(1/3)Co_(1/3))O₂ inExample 1;

FIG. 9 is a graph showing cycling stability of a coin cell inExperimental Example 2, comprising TR01 sample as a cathode activematerial;

FIG. 10 is a graph showing cycling stability of a coin cell inComparative Example 1, comprising inexpensive LiCoO₂ as a cathode activematerial;

FIG. 11 is a graph showing cycling stability of a polymer cell inExperimental Example 3, comprising a cathode active material of TR01(23° C., 45° C., 1 C rate (discharge)-0.6 C rate (charge), 3.0 V to 4.2V, 3.0 V to 4.3 V, 3.0 V to 4.35 V, 3.0 V to 4.4 V, 400 cycling);

FIG. 12 is an FESEM image which shows a precursor sample (Precursor 1)and the resulting electrode active material (4 kg-Final) in Example 2;and

FIG. 13 a graph showing cycling stability of electrode active materialin Example 2, wherein the core of LiCoO₂ is fully covered by the shellof solid state lithium buffer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with an aspect of the present invention, the above andother objects can be accomplished by the provision of a powdercomprising LiCoO₂ having a stoichiometric composition via heat treatmentof a lithium cobalt oxide and a lithium buffer material to makeequilibrium of a lithium chemical potential therebetween.

In accordance with another aspect of the present invention, there isprovided a LiCoO₂-containing powder having a constant lithium chemicalpotential, wherein LiCoO₂ coexists with a lithium buffer material underequilibrium conditions of the lithium chemical potential therebetween,and the lithium chemical potential of powder is higher than theequilibrium lithium chemical potential between LiCoO₂ and a cobalt oxideand is lower than the equilibrium lithium chemical potential betweenLiCoO₂ and Li₂CO₃.

In accordance with a further aspect of the present invention, there isprovided a lithium buffer material which acts as a Li acceptor or Lidonor to thereby remove Li-excess or supplement Li-deficiency, whichcoexists with a stoichiometric lithium metal oxide.

In accordance with a still further aspect of the present invention,there is provided a method for preparing a LiCoO₂-containing powder,comprising a step of providing a homogeneous mixture of LiCoO₂ and alithium buffer material; and a step of heating the resulting mixture tomake an equilibrium of a lithium chemical potential.

In accordance with yet another aspect of the present invention, there isprovided an electrode comprising the above-mentioned LiCoO₂-containingpowder as an active material, and a rechargeable battery comprising thesame electrode.

Hereinafter, the present invention will be described in more detail.

<Lithium Chemical Potential>

Generally, the chemical potential can be defined as the changes in acharacteristic thermodynamic state functions (internal energy, enthalpy,Gibbs free energy, and Helmholtz free energy) with respect to the changein the number of moles of a particular constituent, depending on theexperimental conditions. The chemical potential can be expressed by thefollowing equation, under the conditions of constant temperature andconstant pressure:

$\mu_{i} = ( \frac{\partial G}{\partial N_{i}} )_{T,p,N_{j \neq i}}$

wherein μ is a chemical potential, G is Gibbs free energy, N is thenumber of molecules, T is a temperature and p is pressure.

Therefore, the chemical potential is regarded as the energy state ofeach material in the specific system. If two different materials withdifferent chemical potential coexist in the same system, the reactiontakes place in order to lower the sum of potentials, and the twomaterials equilibrate to the same potential.

In the present invention, “i” is lithium, “j” is oxygen, but by aspecial thermodynamic operation called Lagrange transformation “j” isreplaced by p(j) which is the oxygen partial pressure. Other “j” (Mn,Co, Ni.) are “frozen”, definition of which will be made hereinafter.

The thermodynamic equilibrium state of two or more differentstoichiometric compounds requires the same chemical potentialtherebetween. If LiCoO₂ coexists with impurity materials (Li₂CO₃ orCoO_(x)), it can be considered that there is the state of chemicalpotential equilibrium between LiCoO₂ and individual impurities. As shownin FIG. 1, Li₂CO₃ has a higher lithium chemical potential than LiCoO₂and CoO_(x) has a lower chemical potential than LiCoO₂. As a result, ifthe composition of LiCoO₂ deviates from the stoichiometry (1:1), eachequilibrium potential profile exhibits a stepwise gradient, not acontinuous form.

Generally, mass-produced LiCoO₂ is somewhat lithium-excess orlithium-deficient, as discussed hereinbefore. The lithium chemicalpotential of such a product has always a higher value equilibrated withLi₂CO₃, or a lower value equilibrated with CoO_(x). Therefore, themass-produced LiCoO₂ is difficult to have a proper value between theupper and the lower chemical potentials (the potential of Li:Co=1:1composition).

LiCoO₂ free of bulk impurity phases (Li₂CO₃ or CoO_(x)) still hassurface defects. This is because defects diffuse out of the crystallitesand accumulate at the surface. As a result, the surface islithium-deficient, then the lithium chemical potential is low.Alternatively, the surface can be lithium-rich, then the lithiumchemical potential is high.

The present invention provides a LiCoO₂-containing powder having aconstant lithium chemical potential, via the heat treatment of LiCoO₂and a material functioning as a Li acceptor and/or a Li donor to bringabout equilibrium of the lithium chemical potential therebetween,whereby the lithium chemical potential of LiCoO₂ is higher than theequilibrium lithium chemical potential between LiCoO₂ and a cobalt oxideand is lower than the equilibrium lithium chemical potential betweenLiCoO₂ and Li₂CO₃.

That is, the phrase “preferred range of a lithium chemical potential ofLiCoO₂” as used herein refers to a chemical potential which is higherthan the equilibrium potential between LiCoO₂ and a cobalt oxide and islower than the equilibrium potential between LiCoO₂ and Li₂CO₃.

If LiCoO₂ having a fixed chemical potential in the preferred range isused as a cathode active material of lithium rechargeable batteries, itis advantageously possible to achieve excellent cycling stability at ahigh voltage.

The lithium chemical potential cannot be easily measured. It is notdirectly related to the open circuit voltage (OCV) of an electrical cellat ambient temperature. The OCV is the lithium potential in a “frozen”cobalt-oxygen lattice framework. As used herein, the term “frozen”refers to a temperature which is low enough to prevent a thermodynamicequilibration within a limited time. The entropy of a crystallinesubstance is zero at the absolute zero of temperature (0 K), therebyexhibiting completely different thermodynamic behavior.

Contrary, the lithium chemical potential at room temperature isdominated by the transition metal composition and the lithiumstoichiometry, and furthermore it is related to the conditions duringpreparation.

As discussed before, the performance properties of commercial LiCoO₂ asthe electrode active material depend very sensitively on the exact Li:Coratio. Upon slight deviation from the exact Li:Co ratio, the strongchange in properties of LiCoO₂ is caused by the step-like change oflithium chemical potential. Accordingly surface properties, whichdominate the storage and high-voltage cycling properties, changestepwise as well.

Obviously, it would be preferable to eliminate the step-like change ofthe lithium potential, and to fix the potential within a preferredregion. Then a small deviation of lithium stoichiometry away from theoptimum stoichiometric value would only cause a small change of thelithium chemical potential, as a result surface properties would be onlyscatter slightly from the optimum and generally, a more robust cathodematerial less sensitive to changes of the composition is achieved. Sucha robust cathode material can then be prepared at high quality and lowcost with less requirements regarding perfect process control andquality management as will be disclosed in the following.

<A Method for Equilibrating the Lithium Chemical Potential by thePresent Invention>

FIG. 2 shows an illustrative example explaining some basic thermodynamicproperties of a solid state lithium buffer such asLiMn_(1/3)Ni_(1/3)Co_(1/3)O₂.

Being a buffer means that it can act as a lithium acceptor and/or alithium donor, and that the lithium chemical potential varies littlewith lithium stoichiometry, and that the absolute value of a slope of achemical potential versus lithium stoichiometry is much smaller thanthat of lithium metal oxide which would be removed or supplemented withLi.

If the Li:M ratio in LiMO₂ as the solid state lithium buffer is 1:1,basically Mn is tetravalent, Co is trivalent and Ni is divalent. If thesample is lithium-rich, Li_(1+x)M_(1−x)O₂, a portion of Ni has atrivalent state. If the sample is lithium-deficient, Li_(1−x)M_(1+x)O₂,a portion of Mn changes from 4 to 3 valent state, or a portion of Cochanges from 3 to 2 valent state. The changes of valence state occurwithin the same crystallographic layer structure and enable achievementof a wide stoichiometric range. Because of the wide stoichiometricrange, the changes of lithium chemical potential within a limited regionare not significant.

A mixture of LiCoO₂ with a solid state lithium buffer such as LiMO₂ (forexample, M=Mn_(1/3)Ni_(1/3)Co_(1/3)) does not react (i.e. it does notequilibrate) at ambient temperature. Therefore, all LiCoO₂ particles inthe mixture maintain a low or high lithium chemical potential, and thebuffer particles have a potential determined by the lithiumstoichiometry of the buffer.

During heating of the mixture a reaction takes place as follows. At amedium temperature, possibly above 200° C., the lithium becomes verymobile. This is not sufficient for the equilibration of the lithiumchemical potential because the buffer reaction involves a change oftransition metal valence state. This is also accompanied by an uptake orrelease of gas phase oxygen. At a higher temperature, possibly above400° C., the oxygen becomes mobile, but the transition metal cations arestill frozen. Now the lithium chemical potential and the oxygenpotential equilibrate. At that temperature, the buffer can consumelithium by decomposition of Li₂CO₃ impurities, or it can release lithiumto lithiate the cobalt oxide impurities. Finally, the lithium chemicalpotential of LiCoO₂ equilibrates at the buffer potential.

At much higher temperatures (>>1000° C.), the transition metal cationsbecome mobile, and can therefore react with LiCoO₂ to form a newmaterial.

The present invention discloses LiCoO₂ which is co-fired with a lithiumbuffer serving as the Li acceptor or Li donor, at a temperature above400° C., i.e. at a temperature which is high enough to achieve anequilibration of lithium and oxygen. Otherwise the temperature is lowenough (below 1000° C.) so that the transition metal has not fullyequilibrated. As a result, stoichiometric LiCoO₂, free of Li₂CO₃ orcobalt oxide impurities and coexisting with the lithium buffer, isachieved. The lithium chemical potential of LiCoO₂ is fixed at thelithium buffer potential. The buffer is chosen so that the lithiumpotential of LiCoO₂ is fixed within a preferred region.

FIG. 3 is a schematic diagram which shows a thermodynamic equilibriumstate of a mixture of LiCoO₂ (having a small lithium excess) with alithium buffer LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂. Initially, the lithiumpotential is different in LiCoO₂ and buffer. During the equilibrationprocess, the buffer consumes lithium (by decomposition of the Li₂CO₃impurities) until stoichiometric LiCoO₂ is achieved, and the lithiumchemical potential is equilibrated and fixed within the preferredregion.

FIG. 4 is a schematic diagram which shows a thermodynamic equilibriumstate of a mixture of lithium-deficient LiCoO₂ with a lithium bufferLiMn_(1/3)Ni_(1/3)Co_(1/3)O₂. Initially, the lithium potential isdifferent in LiCoO₂ and buffer. During equilibration, the bufferdelivers lithium (to lithiate the cobalt oxide) until stoichiometricLiCoO₂ is achieved, and the lithium chemical potential is equilibratedand fixed within the preferred region.

<Selection of Solid State Lithium Buffer Material>

Lithium buffer materials may serve as lithium acceptors and/or lithiumdonors. Lithium donating properties are required to lithiate CoO_(x)impurities to form LiCoO₂. Lithium donators are known in the art.Non-limiting examples of the lithium donors are lithium-containingoxides such as Li₂MnO₃. Otherwise lithium accepting properties arerequired to decompose excess Li₂CO₃ impurities. Mild lithium acceptors,strong enough to decompose Li₂CO₃ but not strong enough to delithiateLiCoO₂, are also known in the art. Non-limiting examples of the lithiumacceptors may include TiO₂ (reacting to form Li₂TiO₃), ZrO₂ (→Li₂ZrO₃),Al₂O₃ (→LiAlO₂), MnO₂ (→Li₂MnO₃), LiMn₂O₄ (→Li₂MnO₃), etc. Thesecompounds can be generalized as oxides which are able to form doubleoxides with Li₂O.

Other examples of lithium-accepting compounds donate anions, preferablyfluorine or phosphate ions, which trap excess lithium thereby formingstable lithium salts. Non-limiting examples of such compounds are MgF₂(→2LiF+MgO), Li₃AlF₆, AIPO₃ (→Al₂O₃+Li₃PO₄) and transition metal-basedphosphates (such as Co₃(PO₄)₂ and LiCoPO₄) etc. Such lithium acceptorsmay be effective to decompose Li₂CO₃ impurities, but they cannotlithiate CoO_(x) impurities. Additionally, these compounds areelectrochemically inert, that means they do not contribute to thereversible capacity. Only small amounts of inert compounds, typicallybelow 1% by weight, should be added, otherwise the specific reversiblecapacity of the final cathode will be too low.

The solid state lithium buffer of the present invention preferably has alithium-accepting ability and a lithium-donating ability at the sametime. Preferably, the solid state lithium buffer of the presentinvention also has a high reversible capacity. Preferred examples of thelithium buffers according to the present invention may be lithiumtransition metal oxides of Formula Li_(z)MO₂ (0.95<z<1.1;M=Ni_(1−x−y)Mn_(x)Co_(y), 0<y<0.5, and a ratio of Mn to Ni (x/(1−x−y))is in a range of 0.4 to 1.1).

Specifically, for example, the lithium transition metal composite oxidescomprising nickel, manganese and cobalt, such asLiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ and LiNi_(0.55)Mn_(0.3)Co_(0.15)O₂, arepreferred because they act as lithium donators as well as lithiumacceptors, and additionally, because they can be added in largerquantities (>1% by weight, typically 10% by weight or more) withoutcausing deterioration of the reversible capacity of the final cathodematerial.

In the present invention, an amount of the solid state lithium buffer ispreferably less than 30% by weight of the total weight of the powder.That is, the preferred content of LiCoO₂ in the electrode activematerial is more than 70% by weight.

The minimum amount of the lithium buffer is at least 1 mol %, moretypically at least 10 mol %. For example, if LiCoO₂ has 0.25 mol %impurity of Li₂CO₃, then adding about 1 mol % of LiMO₂, followed byco-sintering yields impurity-free LiCoO₂ coexisting with lithium-richLi_(1+x)M_(1−x)O₂ (approximate composition Li_(1.2)M_(0.8)O₂).

Alternatively, a proper amount of lithium-deficient Li_(1−x)M_(1+x)O₂may be added as the solid state lithium buffer. Alternatively, adding 10mol % of LiMO₂ to LiCoO₂ with 0.25 mol % impurity of Li₂CO₃ would resultin LiCoO₂ coexisting with slightly lithium-rich Li_(1+x)M_(1−x)O₂(approximate composition Li_(1.025)M_(0.975)O₂).

It is recommended to add a sufficient amount of the solid state lithiumbuffer (or to adjust the initial lithium stoichiometry) to achieve apreferred lithium stoichiometry of the buffer after co-sintering. If thelithium content of the lithium buffer in the final product is too low ortoo high, then generally the electrochemical properties of the buffer(for example, reversible capacity) are less.

For example, if LiMO₂ with M=Mn_(1/3)Ni_(1/3)Co_(1/3) orM=Ni_(0.55)Mn_(0.3)Co_(0.15) is used as the lithium buffer material, thelithium stoichiometry of the buffer after the heat treatment should bewithin a desired range otherwise the electrochemical propertiesdeteriorate. Slightly lithium-rich Li_(1+x)M_(1−x)O₂ (x<0.04) andstoichiometric LiMO₂ as wells as slightly lithium-deficientLi_(1−x)M_(1+x)O₂ (x<0.03) are also within this preferred range. Eventhough the above-mentioned preferred range cannot be exactly defined, itseems that the value of x=−0.03, +0.04 in the final lithium buffer isdefinitely within this range.

It is preferred to ensure that the lithium chemical potential of thesolid state lithium buffer matches the lithium chemical potential ofLiCoO₂ and is in the preferred range which is higher than theequilibrium potential between LiCoO₂ and a cobalt oxide and is lowerthan the equilibrium potential between LiCoO₂ and Li₂CO₃. As an example:The preferred Li:M ratio for LiMO₂ with a high content of nickel (>80%)is 1:1. However, at this composition the lithium chemical potential istoo high. Otherwise, the lithium chemical potential of lithium manganesespinel is lower than the equilibrium potential between LiCoO₂ and thecobalt oxide corresponding to a lower limit of the above-mentionedoptimal range. Therefore, spinel is a too strong lithium acceptor whichwill decompose LiCoO₂.

The electrode active material in the present invention is not limited toa specific form as long as LiCoO₂ contacts with the lithium buffer(material serving as the lithium acceptor and/or the lithium donor). Inthe simplest case, the electrode active material is in the powder form.Typically, the LiCoO₂ powder and the solid lithium buffer powder aremixed, followed by heat treatment (co-firing).

In one preferred embodiment, the heat-treated mixture is a co-firedmixture of an oxide powder (a) of LiCoO₂ and a lithium transition metaloxide powder (b) of Formula Li_(z)MO₂ (0.95<z<1.1;M=Ni_(1−x−y)Mn_(x)Co_(y), 0<y<0.5, and a ratio of Mn to Ni (x/(1−x−y))is in a range of 0.4 to 1.1). Herein, the oxide powder (a) is monolithicparticles having D50 of more than 10 μm, and the oxide powder (b) isagglomerated particles having D50 of less than 10 μm.

Generally, where a particle size of the electrode active material islarger, this may lead to a decrease in a surface area for reaction withan electrolyte inside a battery cell, thereby causing significantdeterioration of high-voltage storage properties and rate properties andconsequently decreasing a particle diameter of the active material. Onthe other hand, the electrode active material with a large particlediameter exhibits relatively high structural stability includinghigh-temperature properties and decrease of side reactions includingelectrolyte decomposition, as compared to the active material having asmall particle diameter.

However, it was surprisingly confirmed that the co-fired mixtureaccording to the present invention maintains a desired level ofexcellent high-voltage storage properties, even though the oxide powder(a) has a large particle diameter of more than 10 μm. As describedbefore, this is because LiCoO₂ having a stoichiometric composition or amixture having a proper lithium chemical potential can be obtained, dueto buffering effects of the Li_(z)MO₂ powder (b) as the lithium buffermaterial, on the LiCoO₂ powder (a), during heat treatment.

Alternatively, the solid state lithium buffer can be added in powderform, dispersed in a solution, or it can be dissolved in the solution.

Instead of a simple mixing of powders, more complex preparation routesmight allow not only to improve cycling and storage properties, but alsoachieve improvement of the safety. Generally, it is known that LiCoO₂has poor safety and Mn-containing LiMO₂ has better safety. Therefore, ifa portion of the buffer covers the LiCoO₂ surface, the safety could beimproved.

For example, fine particles of LiMO₂ may be coated on the surface of theLiCoO₂ particles. A typical method would involve spray-coating ordry-coating of small, preferably monolithic LiMO₂ particles (1-3 μm)onto larger LiCoO₂ particles (5-20 μm). During sintering, not only theLi₂CO₃ and CoO_(x) impurities are consumed, and the lithium chemicalpotential is fixed within the preferred region, but also the small LiMO₂particles become strongly attached to the LiCoO₂ surface and effectivelycover a large fraction of the LiCoO₂ surface.

Alternatively, a layer of transition metal hydroxide or transition metalcarbonate can be coated onto the LiCoO₂ by precipitation, using theLiCoO₂ particles as seeds. After eventually adding further lithium andsintering, not only the impurities are consumed, and the lithiumchemical potential is fixed within the preferred region, butadditionally a dense thick layer of electrochemically active LiMO₂effectively covers the LiCoO₂ surface.

<Heat-Treatment>

After addition of the lithium buffer to LiCoO₂, a heat treatmentfollows. In some cases, additional additives may be added before theheat treatment. The additives may be sources of additional lithium (suchas Li₂CO₃ and LiOH.H₂O), or the additives may extract lithium and supplyfluorine (such as MgF₂ and Li₃AlF₆), or the additives may be materialssuitable to modify the surface of the particles (for example, sinteringagents)

The heat treatment typically is made in air, or alternatively it can becarried out in controlled, oxygen-containing gases with a poor oxygencontent or in mixed gas of oxygen and nitrogen.

The heat treatment is carried out at a proper temperature. A suitabletemperature range for the heat treatment is 400 to 1100° C., morepreferably 500 to 950° C. A temperature of less than 400° C. might betoo low. At such a low temperature, the equilibration of lithiumpotential between the lithium buffer and the LiCoO₂ may require anunreasonable long time. If the temperature exceeds 500° C., theequilibration of lithium potential between LiCoO₂ and lithium buffer andalso the necessary equilibration of the oxygen potential usually occurat reasonable kinetics. If the heat treatment is carried out at a veryhigh temperature exceeding 1100° C., this may undesirably result insignificantly increased process costs. That is, the heat treatment atthe very high temperature suffers from high costs for installation ofhigh temperature equipment, large consumption of energy, and a need foradditional processing steps such as grinding and sieving of sinteredcakes. Additionally, at such temperatures not only the lithium andoxygen potentials equilibrate, but also the transition metal diffusionbecomes significant, thus resulting in a doped LiCoO₂.

If the lithium buffer is LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ for example,dominantly cobalt from the LiCoO₂ would diffuse into the LiMO₂, and lessMn—Ni from the LiMO₂ would diffuse into the LiCoO₂, resulting in a phasemixture of two LiCo_(1−x)(Mn—Ni)_(x)O₂ phases, one with x>0.333 and theother with x≅0.

<Preparation of Electrode and Rechargeable Batteries>

An electrode comprising the LiCoO₂ material according to the presentinvention as an electrode active material can be prepared by aconventional method known in the art. For example, the electrode in thepresent invention may use an electrical conducting material forproviding electric conductivity, and a binder for adhesion between theelectrode material and a current collector, in addition to such anactive material.

A paste is prepared by mixing the above-prepared electrode activematerial, 1 to 30 wt % of the electrical conducting material and 1 to 10wt % of the binder in a dispersion solvent, followed by stirring. Alaminated electrode structure is prepared by applying the resultingelectrode paste to a metal sheet current collector, and pressing anddrying the resulting structure.

A general example of the electrical conducting material is carbon black.The products sold in market may include various acetylene black series(available from Chevron Chemical Company and Gulf Oil Company), KetjenBlack EC series (available from Armak Company), Vulcan XC-72 (availablefrom Cabot Company) and Super P (available from MMM Company).

Representative examples of the binders may includepolytetrafluorethylene (PTFE), polyvinylidene fluoride (PVdF) or acopolymer thereof, and cellulose.

Representative examples of the dispersion solvents may include isopropylalcohol, N-methylpyrolidone (NMP) and acetone.

The metal sheet for the current collector should be a highly electricalconductive metal to which the paste can be easily attached. Further themetal sheet should be non-reactive in the range of a working voltage ofcells. If that condition is satisfied, any metal sheet can be used.Representative examples of the metal sheets may be mesh or foil ofaluminum or stainless steel.

The present invention provides a rechargeable battery comprising theelectrode of the present invention. The rechargeable battery of thepresent invention can be prepared by a conventional method known in theart, which is not particularly limited. For example, the battery can befabricated by interposing a separator between the cathode and the anodeand introducing a non-aqueous electrolyte into the resulting electrodeassembly. The electrode, separator, electrolyte, and optionallyadditives known in the art can be used.

A porous separator can be used as a separator upon fabrication of thebattery. Specific examples of the porous separator may include, but arenot limited to, polypropylene series, polyethylene series, andpolyolefin series.

The non-aqueous electrolyte for the rechargeable battery of the presentinvention contains a cyclic carbonate and/or a linear carbonate.Examples of the cyclic carbonate may include ethylene carbonate (EC),propylene carbonate (PC), and gamma butyrolactone (GBL). Examples of thelinear carbonate may include diethyl carbonate (DEC), dimethyl carbonate(DMC), ethylmethyl carbonate (EMC), and methylpropyl carbonate (MPC).The non-aqueous electrolyte for the rechargeable battery of the presentinvention contains a lithium salt in conjunction with the carbonatecompound. Specific examples of the lithium salt may include LiClO₄,LiCF₃SO₃, LiPF₆, LiBF₄, LiAsF₆ and LiN(CF₃SO₂)₂.

EXAMPLES

Now, the present invention will be described in more detail withreference to the following examples. These examples are provided onlyfor illustrating the present invention and should not be construed aslimiting the scope and spirit of the present invention.

Reference Example 1 Preparation and Characterization of LiCoO₂ withLi-Excess or Li-Deficiency

This reference example is intended to demonstrate that anelectrochemical property of LiCoO₂, particularly the cycling stabilityat a high voltage, sensitively depends on the Li:Co ratio.

Commercially available LiCoO₂ was purchased from a low-cost supplier andused as a precursor material for this experiment. Three LiCoO₂ samples(Li⁻, Li0 and Li⁺) were prepared from the precursor. Samples Li⁻ and Li⁺were in an amount of about 1 kg, and Sample Li0 was approx. 100 g.

The lithium-deficient sample Li⁻ was prepared by immersing 1 kg ofLiCoO₂ into water, resulting in a total of 2 L slurry. 7.6 g ofconcentrated Li₂SO₄ was added with stirring the slurry. Three differentreactions took place after addition of acid: (1) decomposition of Li₂CO₃impurities, (2) dissolution of divalent cobalt, and (3) ion exchange(ion leaching) of Li ions with H⁺ ions on the surface region of LiCoO₂particles. Initially the pH dropped to about 2, but slowly increased toabout 6 after 30 minutes. The powder was recovered by filtration. Afterdrying the powder at 180° C., the above procedure was repeated.

ICP analysis of the thus-filtered solution showed that total approx. 2.5mol % lithium and 0.6 mol % cobalt were extracted from LiCoO₂. In thismanner, a lithium cobalt oxide (LiCoO₂) with an approximate compositionof Li_(0.98)CoO₂ was obtained. The lithium-deficient Li_(0.98)CoO₂ washeated at 750° C. for 10 hours.

The approx. stoichiometric sample, Li0, was prepared by heating theprecursor LiCoO₂ at 850° C. for 10 hours. A small amount (100 g) of asample was used to allow for evaporation of eventual excess lithiumimpurities.

The lithium-rich sample Li⁺ was prepared from 1 kg of inexpensive LiCoO₂by adding 1.5 mol % ball-milled LiOH.H₂O per 1 mol cobalt followed by aheat treatment at 750° C. for 10 hours in air.

X-ray diffraction (XRD) analysis showed that all of 3 samples basicallyhad the same XRD pattern. Particularly, the lattice constants wereidentical therebetween. The pH titration of the samples Li⁻, Li0 and Li⁺revealed that Li⁻ and Li0 were basically free of Li₂CO₃ impurities,whereas Li⁺ contained about 1% by weight of Li₂CO₃. Sample Li⁺ would notbe suitable for commercial batteries because the Li₂CO₃ impurities wouldcause un-acceptable amounts of gas (for example causing swelling ofpolymer cells) during charging the battery.

The samples were subjected to electrochemical tests by coin cells at3.0-4.2, 3.0-4.4 or 3.0-4.5 V and at room temperature (25° C.) orelevated temperature (50° C.). A typical schedule involved 32 cycles: acharge rate was C/5. During Cycles 1-5, a discharge rate performance wasmeasured (C/10, C/5, C/2, 1 C and 2 C). Cycles 6-30 were carried out ata C/5 discharge rate to investigate the cycling stability. Cycle 31 wascarried out at a C/10 discharge rate to investigate the remainingcapacity, and Cycle 32 was carried out at a C/1 discharge rate tomeasure the capacity loss (impedance built-up) at a high-rate discharge.

All samples showed an excellent cycling stability at 4.2 V, butexhibited a strong capacity fading at 4.5 V, especially for theLi-deficient sample. Significant impedance built-up was observed in thelithium-deficient and lithium-rich samples. Details are shown in FIG. 5,and Table 1 below summarizes the results.

TABLE 1 Capacity (mAh/g) Capacity fading Sample 25□, C/5 (% per 100cycles) (Li:M 4.2 V 4.5 V 4.2 V 4.5 V target) 25□ 50° C. 25° C. 50° C.Comment Li:M Li⁻ 136 186 2.5 >70 Poor rate Too low (0.98:1) Li0 138 1862.2 8.4 — — (1.00:1) Li⁺ 139 183 2.2 >50 — Too high (1.02:1)

Discussion: The data show that the cycling stability of LiCoO₂ at a highvoltage dramatically changes even with a slight changes of a Li:Coratio. The high-voltage cycling stability (and storage properties athigh temperatures) is dominated by surface properties. The surfacechemical properties depend on the chemical potential. Because thelithium chemical potential changes stepwise, the high-voltage cyclingstability also changes stepwise. If the lithium chemical potential isfixed within a preferred region (according to the present invention),the high-voltage cycling stability can be improved.

Reference Example 2 Properties of Lithium BufferLiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ as Li Acceptor

The reference example is intended to confirm thatLiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ is a lithium acceptor.

This is proven by mixing LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ with a smallamount of Li₂CO₃ (total Li:M=1.025:1), followed by a cooking step at900° C. X-ray diffraction (XRD) analysis showed the following results.

(1) All Li₂CO₃ was consumed. This fact was also confirmed by pHtitration. That is, after immersing the sample into water, the remainingLi₂CO₃ impurities were dissolved in water and detected by pH titration.

(2) The lattice constants (a_(hex), c_(hex), and the unit cell volume)of the final sample (2.8602 Å, 14.2302633 Å, and 33.60586 Å) weresmaller than those of the initial LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ (2.8620Å, 14.23688 Å and 33.66297 Å). These results confirm that lithiumoriginating from the Li₂CO₃ has been introduced into the crystal latticestructure, resulting in Li_(1+x)M_(1−x)O₂. (see also Reference Example 4for the relationship between stoichiometry and lattice constant).

Discussion: If LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ is added to LiCoO₂ withsmall amounts of Li₂CO₃ impurities, followed by co-sintering, then theLi₂CO₃ impurities are consumed, the LiMO₂ buffer is lithiated to giveLi_(1+x)M_(1−x)O₂ and the lithium chemical potential of LiCoO₂ is fixedat the same value as Li_(1+x)M_(1−x)O₂ which is below the high value forLiCoO₂ coexisting with Li₂CO₃.

Reference Example 3 Properties of Lithium BufferLiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ as Li Donor

This example is intended to demonstrate thatLiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ is not a overly strong lithium acceptor andcan also act as a lithium donor.

LiCoO₂ and MOOH (M=LiMn_(1/3)Ni_(1/3)Co_(1/3)) were mixed in a 5:3ratio. The resulting mixture was pressed into the pellets. Afterco-sintering of the pellets at 800° C. for 1 day, X-ray diffraction(XRD) analysis was carried out on the sintered materials and Rietveldrefinement was made. The refinement yielded the following conclusions:

1) Co₃O₄, LiCoO₂ and Li-M-O₂ coexist.

2) The lattice constants and the unit cell volume of the final Li-M-O₂were slightly larger than those of LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂. Thatis, the final Li-M-O₂ exhibited a small lithium deficiency. Using thedata of Reference Example 4 allows to estimate the composition asapprox. Li_(1−x)M_(1+x)O₂ with x≈4.025 (Li:M≈0.95).

Applying basic thermodynamic considerations confirms that the lithiumchemical potential of LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ is above the lowvalue of LiCoO₂ coexisting with cobalt oxide impurities. Conclusion:LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ acts as the lithium donor to lithiate Co₃O₄impurities.

Reference Example 4 Relationship Between the Stoichiometry and theCrystal Lattice of Lithium Buffer LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂

This reference example is intended to investigate thestoichiometry-lattice relation of the lithium bufferLiMn_(1/3)Ni_(1/3)Co_(1/3)O₂.

Commercial LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ with an approximate Li:M ratioof 1:1 was used as a precursor. Two lithium-rich samplesLi_(1+x)M_(1−x)O₂ with target Li:M=1.025:1 and 1.075:1 were prepared byadding Li₂CO₃ to the precursor material and reacting the resultingmixture at 900° C. for 24 hours in air. Two lithium-deficient samplesLi_(1−x)M_(1+x)O₂ with target Li:M=0.975:1 and 0.925:1 were prepared byadding a mixed hydroxide (MOOH, M=Mn_(1/3)Ni_(1/3)Co_(1/3)) to theprecursor material and reacting the resulting mixture at 900° C. for 24hours in air.

X-ray diffraction (XRD) analysis showed a gradual and smooth change of acrystal lattice constant as a function of lithium stoichiometry. Dataare given in Table 2 below. The same data are also shown in FIG. 6.

TABLE 2 Prepara- Li:M tion hex a (Å) hex c (Å) c: a/√24 Vol (Å³) 0.925:1Added 2.8642 14.251 15.65043 33.74779 MOOH 0.975:1 Added 2.8632 14.2408415.2743 33.70063 MOOH    1.0:1.0 As 2.8620 14.23688 15.41883 33.66297received 1.025:1 Added 2.8602 14.23026 15.57188 33.60586 Li₂CO₃ 1.075:1Added 2.8575 14.22082 15.84287 33.52114 Li₂CO₃

The above results of Reference Examples 2-4 confirm those of theschematic FIG. 2 to FIG. 4.

Conclusion: LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ is a suitable solid statelithium buffer. It is able to donate as well as to accept lithium. Thebuffer potential matches LiCoO₂ potential and is within the preferredregion. It has a wide non-stoichiometric range.

Reference Example 5 Electrochemical Properties of Lithium BufferLiMn_(1/3)Ni_(1/3)Co_(1/3)O₂

This example discloses further properties of the solid state lithiumbuffer LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂.

Electrochemical properties of the samples of Reference Example 4 weretested. Coin cells (with a Li metal anode) were prepared and were testedat 25° C. and 60° C. The charging voltage was 4.4, 4.5 and 4.6 V. Table3 below summarizes the obtained results. FIG. 7 shows some of theresults.

TABLE 3 Capacity Sample (mAh/g) Fading @ 60° C. (Li:M 3.0-4.4 V, C/5 %per 100 cycles target) 25° C. 60° C. 4.4 V 4.5 V 4.6 V Comment Li:M0.925:1 164 179 8.4 18 56 Low rate Too low 0.975:1 167 180 4.7 13 53 —OK    1:1 167 179 4.6 11 54 — OK 1.025:1 168 178 4.8 10 56 — OK 1.075:1163 174 10.9 22 75 High Too high fading

Within a relatively broad preferred region (about 0.975:1 to 1.025:1)excellent cycling stability was achieved. The sample with highlithium-deficiency (0.925:1) showed some deterioration of rateperformance. Samples with a low or high Li:M ratio (0.925:1 or 1.075:1)showed some deterioration of cycling stability.

Discussion: The relatively broad preferred region, and the smooth changeof electrochemical properties are caused by the gradual change oflithium chemical potential. Further, other properties including surfacechemistry (by pH titration) were checked. A similar slight andcontinuous variation of properties depending on the Li:M ratio wasobserved.

Example 1 Preparation of LiCoO₂ with Fixed Lithium Chemical Potential

3.6 kg of inexpensive LiCoO₂ (received from a low-cost producer) and 400g of commercial LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ were mixed by slow ballrolling, followed by a co-sintering at 900° C. for 10 hours in air. Thesamples was filled into vials (no sieving or grinding was required)shortly after cooling down and was stored and further processed in a dryroom.

FIG. 8 shows FESEM micrographs of the LiCoO₂ precursors and the finalsample (Sample name: TR01). The morphology of TR01 was just the same asa mixture of the precursors. Particularly, LiCoO₂ andLiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ were still separated particles and were notsintered into agglomerates. A temperature of 900° C. is high enough fora fast equilibration of lithium and oxygen chemical potentials.Therefore, the lithium potential of LiCoO₂ is fixed at a preferredvalue, which is determined by the lithium chemical potential of thelithium buffer. The value is above the low value for LiCoO₂ coexistingwith the cobalt-oxide, and below the upper value for LiCoO₂ coexistingwith Li₂CO₃. Furthermore, the LiCoO₂ was basically free of the cobaltoxide or Li₂CO₃ impurities. The absence of Li₂CO₃ impurities wasconfirmed by pH titration.

Experimental Example 1 Effects of Co-Firing

In order to confirm effects of heat-treatment in the present invention,the electrochemical properties of TR01 prepared in Example 1 werecompared with a sample which is a mixture of 90 wt % of heated LiCoO₂and 10 wt % of LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂.

The sample was prepared in the same manner as in Sample TR01, exceptthat two materials were not heat treated (co-fired). Table 4 belowsummarizes the results.

TABLE 4 Capacity mAh/g) Capacity fading at C/5, (% per 100 cycles)Samples 4.5 V, 4.5 V 4.5 V (Li:M target) 50° C. 25° C. 50° C.LiCoO₂—LiMO₂ mixture 190 12% 31% TR01 (Example 1) 187 2.0%   6%

The results of Table 4 showed that a simple mixing of LiCoO₂ withLiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ without heat-treatment is not sufficient toachieve a high cycling stability at a high voltage.

Conclusion: A heat treatment is required to achieve equilibration of thelithium chemical potential.

Experimental Example 2 Coin-Cell Test

A cathode was prepared using Sample TR01 prepared in Example 1, as acathode active material. Coin cells (with a Li metal anode) wereprepared and were tested at 4.4 V and 4.5 V and at 25° C. and 50° C.FIG. 9 and Table 5 below summarize the obtained results.

TABLE 5 Capacity (mAh/g) Capacity fading at C/5 % per 100 cycles Sample4.4 V 4.5 V 4.4 V 4.5 V 4.5 V (Li:M target) 25° C. 50° C. 50° C. 25° C.50° C. TR01 171 187 2.5% 2.0% 6%

From the results given in Table 5 and FIG. 9, it can be seen that SampleTR01 of Example 1 (having the lithium chemical potential fixed in apreferred region) has improved cycling properties at an elevatedvoltage.

Comparative Example 1 Coin Cell Test Using Low-Price LiCoO₂

Except using low-price LiCoO₂ as a cathode active material, a coin cellwas prepared in the same manner as in Experimental Example 2, and testedunder the same conditions. LiCoO₂ used in this Example was the sameproduct which was used as the precursor in Example 1. The low-priceLiCoO₂ powder was heated to 900° C. and refreshed to have the same heattreatment history as Sample TR01 of Example 1. However, the abovecomparative sample was not subjected to the treatment to achieve theequilibration of the lithium chemical potential by the action of thelithium buffer.

As shown in FIG. 10, the comparative sample exhibited excellentstability through the heat treatment, which was, however, lower thanthat of Sample TR01 (see FIGS. 9 and 10).

In addition, it can be seen that the comparative sample exhibited asignificant decrease of the capacity at 4.5 V, 50° C. as well as 4.5V,25° C. (see C/10 data) and much more impedance built-up (see voltagedepression of C/1).

Experimental Example 3 Polymer-Cell Test (Cycling Properties)

Commercial size polymer cells (3.6 mm thickness) were prepared at pilotplant scale. The polymer cells contained a cathode composed of 95 wt %TR01 (Example 1), 2.5 wt % PVDF binder and 2.5 wt % conductive additive(Super P), double side coated on 15 micrometer aluminum foil. The anodeactive material was commercial MCMB (Mesocarbon microbead). A standardcommercial electrolyte (not containing overcharge additives) was used.The anode loading was chosen to achieve balanced cells at 4.4 V cellvoltage (anode capacity=cathode capacity charged to 4.45 V versusLi/Li⁺).

The polymer cells were cycled for 400 cycles (charge rate 0.6 C,discharge rate 1 C, 1 C=800 mA). Each 100th cycle was a “capacity check”cycle during which a slower charge/discharge rate (0.2 C) was applied.The cells were cycled at 3.0-4.2 V, 3.0-4.3 V, 4.0-3.5 V or 3.0-4.35 V.The cycling temperature was 23° C. or 45° C. Frequently the increase ofthickness during extended cycling was checked. Furthermore, theevolution of resistance was checked by impedance measurement at 1000 Hz.

FIG. 11 shows the measurement results of cycling stability at 4.2, 4.3,4.35 and 4.4 V at 23° C. and 45° C. Even at the high cell voltage of 4.4V and at the elevated temperature of 45° C. a very high cyclingstability without significant impedance built-up was observed.

More importantly, at 23° C. a similar rate of capacity loss was obtainedfor all voltages, and additionally, the capacity loss at C/1 and C/5rates evolved similar patterns. Also at 45° C., a similar behavior ofcapacity loss was observed for all voltages. Therefore, it can beconcluded that the increase of cell voltage did not cause degradation ofthe cathode.

Table 6 below summarizes the obtained results.

TABLE 6 Cycle 100 Cycle 200 Cycle 300 Cycle 400 Thickness 23° C. 4.2 V3.474/3.486 3.484/3.495 3.479/3.516 3.475/3.509 (mm) 4.3 V 3.519/3.5363.522/3.533 3.549/3.554 3.562/3.561 2 cells 4.35 V  3.563/3.5673.599/3.624 3.617/3.604 3.628/3.60  each 45° C. 4.2 V 3.538/3.5483.558/3.578 3.571/3.584 3.612/3.637 4.3 V 3.611/3.610 No data3.650/3.648 3.671/3.670 4.35 V  3.607/3.626 No data 3.653/3.6583.689/3.662 Impedance 23° C. 4.2 V 19.6/19.7 20.2/20.5 21.2/21.322.1/22.9 1 kHz 4.3 V 20.8/20.3 22.2/22.0 26.8/26.4 24.8/24.6 mΩ 4.35 V No data 22.9/22.8 25.1/24.5 25.0/24.6 2 cells 45° C. 4.2 V 22.1/22.825.8/26.3 29.0/29.8 31.1/32.3 each 4.3 V 24.6/25.1 No data 33.4/34  37.4/37.8 4.35 V  25.3/25.6 No data 35.8/36.3 38.1/39.7

Conclusion: The obtained results clearly confirm that a modified LiCoO₂,with a lithium chemical potential fixed within a preferred region, hasan improved excellent stability at high voltage (at least up-to 4.4 Vversus Li/Li⁺) even at elevated temperature (45° C).

Experimental Example 4 Polymer-Cell Test (Storage Properties)

Polymer cells, prepared in the same manner as in Experimental Example 3,were charged to 4.2, 4.3 or 4.35 V. After charging, the cells wereplaced in a temperature chamber and the temperature was increased to 90°C. over 1 hour. Cells were kept at 90° C. for 4 hours, and then thetemperature was decreased to room temperature over 1 hour. During thetemperature profile, the cell thickness was automatically monitored.Before and after the test, the cell capacity was measured at C/1 and C/5rates.

No significant increase of thickness was observed at any of theinvestigated charge voltages. Also, the recovery ratio did not decreasewith an increased storage voltage. It can be concluded that the increaseof cell voltage did not cause degradation of the cathode.

Table 7 below summarizes the results.

TABLE 7 Thickness increase Before (mAh) After (mAh) Recovery (%) (μm) 1C 0.2 C 1 C 0.2 C 1 C 0.2 C 4.2 V <40 709 715 673 681 94.9 95.3 4.3 V<30 773 779 739 752 95.7 96.5 4.35 V  <0 795 801 764 779 96.2 97.2

Conclusion: The obtained results clearly confirm that modified LiCoO₂,with a lithium chemical potential fixed at a preferred region, hasimproved, excellent storage properties at a high voltage (at least up-to4.4 V versus Li/Li⁺).

Comparative Example 2 Polymer Cell Test Using Low-Price LiCoO₂

Except using low-price LiCoO₂ as a cathode active material, a polymercell was prepared in the same manner as in Experimental Example 3, andtested under the same conditions as Experimental Examples 3 and 4.However, the cell always showed much inferior stability at >4.3 V andalways showed strong swelling during a 90° C. storage test.

Cells with standard commercial LiCoO₂ exhibited smooth cycling at4.2-4.25 V, but at 4.3-4.35 V an increased rate of capacity loss wasobserved simultaneously with a stronger built-up of capacity difference(=impedance built-up). This behavior was caused by the lack of cyclingstability of LiCoO₂ at voltages>4.3 V versus Li/Li⁺.

Example 2 Preparation of LiCoO₂ with Core-Shell Structure

(1) Experimental Examples 2, 3 and 4 demonstrate that modified LiCoO₂with a lithium chemical potential fixed in a preferred region, obtainedby co-sintering with a solid state lithium buffer, allows to obtainexcellent storage properties and high-voltage cycling properties. Thisactual example modifies this approach. The LiCoO₂ (having a lithiumchemical potential fixed in a preferred region) is present in the coreof particles, covered by a shell of the solid state lithium buffer. Thisconcept is useful to further improve safety properties.

(2) A layer of M(OH)₂ (M=Mn_(1/2)Ni_(1/2)) was precipitated onto LiCoO₂particles acting as seeds during the precipitation process. Duringprecipitation, a flow of an aqueous MSO₄ solution (2M) and a flow of anaqueous NaOH solution (4M) were added to a reactor (5 L) containing 5 kgof LiCoO₂ in the form of a water-based slurry (Volume=2.7 L). The pH wasadjusted to be within a preferred region, and the temperature was about85° C. The total time of precipitation was 165 min. A total of 0.06 moltransition metal was precipitated per 1 mol of LiCoO₂. After theprecipitation, the slurry was filtered and washed, and the resultingpowder cake was aged overnight in 10 L of an aqueous 0.5M LiOH solution,followed by washing and drying at 180° C. The thus-obtained powder(name: Precursor 1) was used as a precursor for the preparation of afinal cathode material.

(3) To find optimum preparation conditions, a series of small scalesamples was prepared and electrochemically tested at 4.4, 4.5 and 4.6 Vand at 25° C. and 60° C., respectively. Samples had varying Li contentsand were prepared by adding small amounts of Li₂CO₃ to Precursor 1,followed by heat treatment at 900° C. for 5 hours. In some cases, asmall amount of fluorine (Li₃AlF₆, 0.2 mol % Al per 1 mol of Co) wasalso added prior to the heat treatment.

(4) Finally, a large sample (volume size: 4 kg, name: 4 kg-Final) wasprepared by adding 48 g of Li₂CO₃ and 20.5 g of a 2:1 mixture of Li₃AlF₆and Li₂CO₃ to 4 kg of Precursor 1, followed by heat treatment at 900° C.for 6 h. Coin cells were prepared using the thus-obtained samples andwere tested at 4.4, 4.5 and 4.6 V at 25° C. as well as at 60° C.

FIG. 12 shows an FESEM micrograph of the thus-prepared cathode activematerial with a core-shell structure. Apparently, a shell of solid statelithium buffer fully covers the LiCoO₂ core. FIG. 13 shows test resultsof the cycling stability. High stability during cycling at high voltageand elevated temperature has been demonstrated. The cycling stabilitywas much improved, as compared with the results of state of the artLiCoO₂.

INDUSTRIAL APPLICABILITY

According to the present invention, LiCoO₂ having a stoichiometriccomposition can be prepared by co-firing with a material acting as a Liacceptor and/or a Li donor, thus fixing a lithium chemical potentialwithin the preferred range. As a result, it is possible to prepare aLiCoO₂ electrode active material which has improved high-temperaturestorage properties and high-voltage cycling properties, and is robust tocomposition fluctuation in the production process.

Therefore, the present invention can spend less time and labor toquality control and process management in the mass-production of theelectrode active material, and the production costs of LiCoO₂ can be cutdown.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A LiCoO₂-containing powder comprising: a lithium cobalt oxide; and alithium buffer material, wherein the lithium buffer material is amaterial of the Formula Li_(z)MO₂, wherein z is between 0.95 and 1.1; Mis Ni_(1−x−y)Mn_(x)Co_(y); y is an amount of up to 0.5; and a ratio of(x/(1−x−y)) is in a range of 0.4 to 1.1, and wherein the lithium cobaltoxide and the lithium buffer material are in equilibrium with respect toa lithium chemical potential therebetween.
 2. The LiCoO₂-containingpowder according to claim 1, wherein the lithium buffer material is amaterial that withdraws Li from Li₂CO₃, which is present in a Li-excessform of LiCoO₂, or supplies Li to a cobalt oxide, which is present in aLi-deficient form of LiCoO₂.
 3. The LiCoO₂-containing powder accordingto claim 1, wherein the lithium buffer material isLiMn_(1/3)Ni_(1/3)Co_(1/3)O₂.
 4. The LiCoO₂-containing powder accordingto claim 1, produced by a method comprising heat-treating at a heattreatment temperature which is lower than a temperature at which theLiCoO₂ and the lithium buffer material form a new compound.
 5. TheLiCoO₂-containing powder according to claim 4, wherein the heattreatment temperature is in the range of 400 to 1100° C.
 6. TheLiCoO₂-containing powder according to claim 1, wherein the content oflithium cobalt oxide is at least 40 weight percent, based on the totalweight of the LiCoO₂-containing powder.
 7. The LiCoO₂-containing powderaccording to claim 1, wherein the lithium cobalt oxide contacts thelithium buffer material.
 8. The LiCoO₂-containing powder according toclaim 1, further comprising a core-shell structure, wherein a corecomprises the lithium cobalt oxide and a shell comprises the lithiumbuffer material.
 9. The LiCoO₂-containing powder according to claim 1,which is prepared on the scale of at least 1 kg batch.
 10. TheLiCoO₂-containing powder according to claim 1, which is prepared on thescale of at least 20 kg batch.
 11. A cathode active material for arechargeable battery comprising the LiCoO₂-containing powder accordingto claim
 1. 12. A LiCoO₂-containing powder comprising: LiCoO₂; and alithium buffer material, and a constant lithium chemical potential,wherein the LiCoO₂ coexists with the lithium buffer material underequilibrium conditions of lithium chemical potential therebetween, andthe lithium chemical potential is higher than an equilibrium lithiumchemical potential between LiCoO₂ and a cobalt oxide and is lower thanan equilibrium lithium chemical potential between LiCoO₂ and Li₂CO₃, andwherein the lithium buffer material is a material of the FormulaLi_(z)MO₂, wherein z is between 0.95 and 1.1; M isNi_(1−x−y)Mn_(x)Co_(y;) y is an amount of up to 0.5; and a ratio of(x/(1−x−y)) is in a range of 0.4 to 1.1.
 13. A lithium buffer material,which acts as a Li acceptor or Li donor to remove a Li-excess orsupplement a Li-deficiency and coexists with a stoichiometric lithiummetal oxide, comprising: a material of the Formula Li_(z)MO₂, wherein zis between 0.95 and 1.1; M is Ni_(1−x−y)Mn_(x)Co_(y;) y is an amount ofup to 0.5; and a ratio of (x/(1−x−y)) is in a range of 0.4 to 1.1. 14.The buffer material according to claim 13, which withdraws Li fromLi₂CO₃, which is present in a Li-excess form of LiCoO₂, or supplies Lito a cobalt oxide, which is present in a Li-deficient form of LiCoO₂.15. A method for preparing a LiCoO₂-containing powder, comprising a stepof providing a homogeneous mixture of LiCoO₂ and a lithium buffermaterial; and a step of heating the mixture to achieve equilibration ofa lithium chemical potential.
 16. The method according to claim 15,wherein the homogeneous mixture has a core-shell structure that thelithium buffer material covers the surface of LiCoO₂.
 17. The methodaccording to claim 15, wherein the heat treatment temperature is lowerthan the temperature at which the reaction between LiCoO₂ and thelithium buffer material takes place to form a new compound.
 18. Themethod according to claim 15, wherein the heat treatment temperature isin the range of 400 to 1100° C.
 19. An electrode comprising the powderclaim
 1. 20. A rechargeable battery comprising the electrode of claim19.